"It may be there isn't a problem," says McBrien. "But we need to assess and make sure how long it can be used for and what effects it has."

He says researchers already know about some of the short-term adverse effects, such as blurred vision and difficulty in focusing.

"If you have someone using a virtual imagery device and it gets their focus adapted in a particular position, they go off and jump in their car and their vision is not perfect, that could have consequences," he says.

A virtual world

In virtual reality devices, an image is projected onto a screen a few millimetres from the eyes. The image appears to be much further away and often in 3D, and gives the person the impression they are in a different reality.

McBrien says virtual reality may confuse the visual system because things that look like they are at different distances away are not really.

For example, when you focus on something close to you, like a piece of paper on your desk, your eyes roll inwards. But if you look out the window, your eyes rotate outwards and your focus changes.

The rotation of the eyes and their focus are normally linked, but in virtual reality they become dissociated.

"That causes a conflict in the visual system," says McBrien, adding that virtual reality also creates other conflicts.

"If you move your head around in a room, the door doesn't move," he says. "When you've got a helmet-mounted [virtual reality] display, if you move your head you will get some movement of the door and that can cause problems as well."

Cumulative effects?

McBrien and others have just reported in a recent issue of the journal Optometry and Vision Science a new questionnaire-based test for studying the impacts of virtual reality.

Of the 13 effects that he has confirmed are due to virtual reality, most did not last more than 10 minutes.

But researchers have yet to study what happens when people use virtual reality technology repeatedly or over the long term.

McBrien also says it will be important to find out why only some individuals are affected.

He adds that children may be more vulnerable to adverse effects from virtual reality because their visual system is still developing.

New devices, new potential impacts

The virtual reality device McBrien used to develop his test was helmet-mounted, which makes the user 'fully immersed' in the virtual world.

This was the first kind of virtual reality device to be developed and now McBrien hopes to check his test on newer devices.

"We've just looked at a device [mounted on] a pair of spectacles that you can play DVDs on and it projects the image about a metre away from you," he says.

Because the person wearing the spectacles can still see the rest of the room at the same time, McBrien says it might be important to look at other possible effects.

"You're presented with a virtual world but you've still got the real world there as well."